Development of Two-electrode Electrogas Arc Welding Process

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NIPPON STEEL TECHNICAL REPORT No. 90 JULY 2004
UDC 621 . 791 . 753 . 9
Development of Two-electrode Electrogas Arc Welding Process
Kiyohito SASAKI*1
Ryu-ichi MOTOMATSU*1
Shigeru OHKITA*2
Kazutoshi SUDA*1
Yuji HASHIBA*2
Shiro IMAI*3
Abstract
Electrogas arc welding (EGW) is used for vertical position welding of sheer strakes
and hatch side coamings of container ships because of its higher welding efficiency.
However, in ordinary welding processes for ultra-thick steel plates, defects such as
the lack of fusion are likely to occur. In order to solve this problem and others, a
two-electrode VEGA® (Vibratory Electrogas Arc Welding) process was developed by
the authors. This paper provides an outline thereto and describes the essential characteristics of this newly developed process for vertical position welding on extraheavy-sectioned steel plates. It was demonstrated that the VEGA® process achieves a
stabilized fusion-line profile for ultra-thick steel plates with higher welding efficiency. It was then confirmed that joint performance in this study satisfies regulations such as class NK KEW53 and KEW53Y40.
(sheer strakes, see Fig. 1) are ultra-thick, it was extremely difficult
to attain an integrity of weld joints and good properties of the welds
using the conventional single pass EGW method. Hatch side coamings
which also is a vertical position welding method (see Fig. 1 for the
1. Introduction
In recent years, there has been a growing trend for ultra-thick,
and higher strength steel plates. This trend is a result of the continuing race for ever larger steel structures. These have resulted in increased demand for higher efficiency of forming, higher quality and
higher strengths of welding materials to handle these increased thicknesses of steel plates.
We have also witnessed a dramatic increase in the sizes of container ships. The increasing sizes are evidence of shipping companies’ need for increased shipping efficiency. Larger container ships
require thicker steel plates and a higher strength. Because vertical
position welding is applied to the sheer strakes and hatch side
coamings on the container ships when they are block-assembled in
the dry dock, the most prevalent method for welding is the electrogas
arc welding (hereinafter referred to simply as EGW) which is capable of a single pass, vertical position welding that is highly efficient.
However, because the steel plates that are used in a ship’s side
1
*
*2
Fig. 1 Cross-section of container vessels
Nippon Steel Welding Products & Engineering Company Limited
Steel Research Laboratories
*
- 67 -
3
Plate Sales Division
NIPPON STEEL TECHNICAL REPORT No. 90 JULY 2004
shows the general concept for the single-electrode VEGA® welding
process.
The development of the single-electrode VEGA® welding process enabled easier formation of top surface beads and back surface
beads. Also, because it was possible to reduce the welding heat input and the amount of weld metal, HAZ toughness and processing
efficiency were dramatically increased. As can be seen in Photo 1,
when the macrostructure of the weld cross-section was compared
with the conventional EGW method and the single-electrode VEGA®
welding process, the welds by VEGA® process showed narrow weld
bead and heat affected zone, even on the same thickness of steel
plate. Also, there was no lack of fusion even for 50 mm steel plates.
Thus it was demonstrated that a sound weld could be attained.
Nevertheless, there is a trend for steel plate thicknesses to increase to 58 mm, and even to 65 mm and 70 mm thicknesses due to
the growth in the size of container ships. Such steel thicknesses are
used in the shear strakes and the hatch side coamings on the container ships. As can be seen in Photo 2, when the thickness of the
steel plate exceeds 50 mm, welding defects (such as a lack of fusion)
are generated even when using the conventional single-electrode
VEGA® welding process. Thus, to handle these increased plate thicknesses, the necessary dwelling time for an arc to fully fuse a groove
edge portion of the hatch) is generally used for even thicker steel
plates. However, because of the application of a 2-pass welding process, worker-hour cost need to be applied has increased. In view of
the conditions described above, a two-electrode VEGA® (Vibratory
Electro-gas Arc Welding) process was developed based on the VEGA®
process which is a simple electrogas arc welding method. The objective of this new process is to stabilize the fusion-line profile for
ultra-thick steel plates and attain higher welding efficiency. This
paper describes how this two-electrode VEGA® welding process was
developed and outlines its features.
2. Conventional Technologies and Related Issues
Degradation of the conventional EGW method1) which was developed in the 1960’s was a problem because of the low impact property of the welds. This was mainly caused because high heat input
was apt to apply and because the wire diameter (2.4 to 3.2 mm) was
thick so the sectional area of groove was wide. This caused the coarse
microstructure of the heat affected zone (HAZ).
To overcome those demerits, a single-electrode VEGA® welding
process was developed in the 1970’s by the Nippon Steel Welding
Products & Engineering Company Limited. This new process enabled a welding with a narrower groove. To reduce the heat input
with the single-electrode VEGA® welding process, a fine diameter
wire was used to lower the welding current and to allow to use a
narrow groove. Also, to attain a sound weld bead formation near the
front and back surfaces of a joint, this welding apparatus is equipped
with a mechanism to oscillate the wire (welding torch) in the plate
thickness direction. Also, provided with a mechanism for holding of
wire extension to a constant as a result that the rising speed of the
carriage is controlled by detecting the welding current. To differentiate with the two-electrode VEGA® welding process which is described below, this single-electrode welding process will hereinafter
referred to as the single-electrode VEGA® welding process. Fig. 2
Photo 1 Comparison of cross-sections of welds by conventional EGW
process and single-electrode VEGA®
Photo 2 Example of “lack of fusion”(indicated by allow) in weld by
single-electrode VEGA® (plate thickness: 70mm)
Fig. 2 Schematic illustration of single-electrode VEGA®
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NIPPON STEEL TECHNICAL REPORT No. 90 JULY 2004
face is insufficient when only using a single-electrode, even if arc
oscillating width is increased. Therefore, welders required an extremely high level of technical skill to weld steel plates that exceeded
thicknesses of 50 mm without the welding defects that are often associated with the single-electrode VEGA® welding process.
3. Development of Two-electrode VEGA® Welding
Process
3.1 Development targets
In order to resolve the issues described above, the authors embarked on efforts to develop the two-electrode VEGA® welding process with the presumption of employing a narrow groove to reduce
heat input during welding. To attain their objective, the number of
electrodes was increased to uniformly distribute welding heat input
in a stable manner in the plate thickness direction. It was supposed
that it would be effective to reduce the width of oscillation for each
of the two electrodes. The following three points were the targets
for the development of the two-electrode VEGA® welding process.
The authors also maintained a view to the need to overcome any arc
interference that could be caused by the application of a second electrode, the overall simplicity of the structure of the apparatus and the
need to optimize welding conditions.
(1) Welding must be possible on steel plates having thicknesses anywhere from 50 mm to 70 mm. There must be no welding defects and joints must be sound.
(2) There must be good mechanical properties of welds.
(3) Good welding workability must be attained and it must contribute to improved efficiency of welding work.
3.2 The two-electrode VEGA® welding process apparatus
In the same way as the conventional single-electrode VEGA®
welding process, the apparatus which is a vertical position singlepass welding method is arranged with a water cooled copper shoe
that automatically rises along with the carriage on the front surface
side of the groove, and a light weight ceramic backing material to
touch the backside of the groove. This was configured to prevent
fusing slag and weld metal from flowing out. Fig. 3 shows the configuration of the 2-electrode VEGA® welding process apparatus; Fig.
4 shows a model thereof.
The two-electrode VEGA® welding process is a simple structure
that applies one more electrode to the single-electrode VEGA® welding process. Mounting and dismounting the welding torch of the
second electrode is simple. Furthermore it is easy to change the
structure into a single or two-electrode apparatus according to the
thickness of the steel plate targeted for welding.
Fig. 4 Schematic illustration of two-electrode VEGA®
The power source for welding was a direct current. This was
selected because of the stability of the arc. If both electrodes have
the same polarity, specifically, if they are both positive or negative,
the arc becomes highly unstable. Accordingly this produces great
amounts of spatter. For this reason, the polarity of the two electrodes was made opposites. Also, normally if the electrode is positive, the beads are wider, and the fusing is shallow. Conversely, if
the electrode is negative the beads are narrow and fusing is deep2).
From these facts, to weld a wide single V groove, the positive electrode was arranged on the grooved surface side because of its ability
for a wide fusing, and the negative electrode was arranged on the
narrow backside of the groove.
3.3 Improved welding workability
The amount of slag generated when welding greatly affects welding workability when using the EGW method. The welding workability described in this report was evaluated according to the amount
of spatter generated, and the external appearance of the beads.
Initially, flux-cored wire which is dedicated to the single-electrode VEGA® welding process was used in both poles for the 2-electrode VEGA® welding process and for the backing material, a product that is supported by the single-electrode VEGA® welding process was used. It was learned that there was a great amount of spatter
generated and that long-term welding would be difficult. A great
amount of fusing slag was built up on the molten pool near the back
side of the groove. On the other hand, the backing material for the
single-electrode VEGA® welding process was provided arc shaped
grooves as shown in Fig. 5 (a) to attain the same kind of excess weld
metal as for general welding. However, this is not a shape that is
appropriate for promoting the discharge of fusing slag that is excessively produced. Two types of backing material were manufactured
with depths of gap to promote discharge of slag (see Fig. 5 (b) and
(c)) with the objective of discharging fused slag that is retained in
Fig. 3 Two-electrode VEGA® apparatus
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NIPPON STEEL TECHNICAL REPORT No. 90 JULY 2004
Fig. 5 Backing materials with various shapes
excess, to the backing material side. Then the welding workability
was evaluated. The welding conditions used are shown in Table 1;
The groove face configuration is shown in Fig. 6; and the results of
the test are shown in photo 3.
The amount of spatter (spattered slag) generated was lower in
the order of the No. 1 configuration then the No. 2 configuration (for
the single-electrode VEGA® welding process) of the backing materials (see Fig. 5 (b) and (c)). A thorough reduction in the amount of
spatter was observed in the process using the shape No. 2. However,
undercuts were generated at the toe of back bead. Because this is
thought to be a cause of fatigue failure, the backing material having
the shape No. 1 was employed.
It is insufficient to reduce the amount of spatter generated for the
markets and the actual process simply by changing the shape of the
backing material. Next, by adjusting the amount of flux of the welded
wire, the amount of slag that is generated from the welding material
Fig. 6 Groove configuration
was reduced. This was used in an attempt to suppress the amount of
slag spatter that is generated. Table 2 shows the welding wire used
in the test, the steel plate and the welding conditions. Note that the
groove face configuration is the same as that depicted in Fig. 6. Table
3 shows the results of evaluations on the welding when using they
flux-cored wire that has half the amount of flux as the wire used with
the single-electrode VEGA® welding process, and a solid wire that
conforms to JIS Z 3325 TGL1-4G (AP). Because it was learned that
the solid wire possesses characteristics having lower slag generating
amounts in comparison to the flux-cored wire, it can be expected to
affect the production of the amount of slag generated on the back
side of the groove where slag retention is high. The study was un-
Table 1 Welding conditions
Electrodes
Welding wire
1st
Flux-cored wire*1
Current
(A)
420
2nd
Flux-cored wire*1
400
*1
: For single-electrode VEGA®
Base plate: EH40 (plate thickness: 70mm)
Voltage
(V)
42
40
Travel
speed
(cm/min)
Heat
input
(kJ/cm)
Electrode
spacing
(mm)
Oscillation
width
(mm)
5.0-5.5
366-404
15
35
Photo 3 Weld test results by various backing materials
- 70 -
Shielding gas
Composition
Flow rate
( /min)
100% CO2
30
NIPPON STEEL TECHNICAL REPORT No. 90 JULY 2004
Test
No.
Electrodes
Welding wire
Current
(A)
420
400
420
400
420
400
420
400
Table 2 Welding conditions
Travel
Heat
Voltage
speed
input
(V)
(cm/min)
(kJ/cm)
42
5.0-5.5
366-404
40
42
5.0-5.5
366-404
40
42
5.0-5.5
366-404
40
42
5.0-5.5
366-404
40
Electrode
spacing
(mm)
Oscillation
width
(mm)
1st
Flux-cored wire*1
15
2nd
Flux-cored wire*1
*1
1st
Flux-cored wire
2
15
2nd
Solid wire*2
*2
1st
Solid wire
3
15
2nd
Solid wire*2
*3
1st
Flux-cored wire
4
15
2nd
Solid wire*2
*1
®
: For single-electrode VEGA
*2
: JIS Z 3325 TGL1-4G (AP)
*3
: For two-electrode VEGA® (contains one-half the flux compared with the wire for single-electrode VEGA®)
Base plate: EH40 (plate thickness: 70mm)
1
Shielding gas
Composition Flow rate
( /min)
35
100% CO2
30
35
100% CO2
30
35
100% CO2
30
35
100% CO2
30
Table 3 Test results with various welding wires
Test
No.
Electrodes
Welding wire
Amount of
spatter
Test results
Bead
Comprehensive evaluation
appearance
for actual constructions
1st
Flux-cored wire*1
Very large
Good
Not suitable
2nd
Flux-cored wire*1
*1
1st
Flux-cored wire
B
Large
Good
Not suitable
2nd
Solid wire*2
*2
1st
Solid wire
C
Very small
No good
Not suitable
2nd
Solid wire*2
*3
1st
Flux-cored wire
D
Small
Good
Suitable
2nd
Solid wire*2
*1
®
: For single-electrode VEGA
*2
: JIS Z 3325 TGL1-4G (AP)
*3
: For two-electrode VEGA® (contains one-half the flux compared with the wire for single-electrode VEGA®)
Base plate: EH40 (plate thickness: 70mm)
A
dertaken with regard to the application of this solid wire to the second electrode which arranged on the narrow backside of the groove.
Case A which employed the flux-cored wire for the single-electrode VEGA® welding process on both electrodes, had the greatest
amount of spatter generated. In the order of B, C, and D, cases C and
D showed that spatter had reduced to an acceptable amount. As a
result, it is suggested that the amount of slag generated from the
welding wire greatly affects the amount of spatter.
Thus, as described above, research was conducted into the backing material configuration and the composition of the welding wire.
Overall evaluations were also conducted on the welding workability. As a result, a decision was made to use the backing material of
the shape No. 1, and to apply the flux-cored wire that halves the
amount of flux as the wire used with the single-electrode VEGA®
welding process on the first electrode and the solid wire on the second electrode. These realized superior welding workability that also
could withstand actual welding work.
good shape, and to form beads on the backside. This development
means that now welders can perform their duties without much worry
with regard to welding defects such as poor penetration or lack of
fusion when changing the oscillating conditions (width of oscillation and stopping time of both ends when oscillating, and position of
aim of the wire) or the gap. Therefore, it can be said that this newly
developed method of welding is highly applicable because it does
not require welders to have high levels of technical training or skill
to weld steel plates that exceed 50 mm in thickness. See photo 4.
This shows sectional views of sample microstructures of welds on
steel plates having 50, 60 and 70 mm thicknesses. Furthermore, the
penetration of weld metal near the front surface, backside surface of
the plate and to the groove face was stabilized allowing for good
quality welding without the defects often associated with welding
such as poor penetration or lack of fusion.
4.2 Improved welding efficiency
Because the 2-electrode VEGA® welding process is substantially
twice the welding speed of the single-electrode welding process,
deposition rate for a single electrode when using the same groove
shape is approximately doubled, as shown in the Fig. 7, thereby demonstrating that this method of applying two electrodes to be highly
efficient. Furthermore, the amount of time required for setup of the
apparatus is substantially the same as that for the single-electrode
VEGA® welding process so efficiency, considering the entire process of welding from set up to actual welding, is substantially twice
that of the single-electrode VEGA® welding process. Still further,
when using the single-electrode, or the 2-electrode VEGA® welding
process, almost all of the welding wire used is contained on single-
4. Results of Application of the Two-electrode
VEGA® Welding Process
By applying a variety of improvements to welding workability, it
has become possible for a highly efficient single pass welding of
steel plates that have thicknesses anywhere from 50 mm to 70 mm
using the 2-electrode VEGA® welding process. The following outlines the effects of this method of welding.
4.1 Stabilized penetration of weld metal
With the development of the 2-electrode VEGA® welding process, it has become possible to easily attain front side beads that have
- 71 -
NIPPON STEEL TECHNICAL REPORT No. 90 JULY 2004
Photo 4 Cross-sections of welds by two-electrode VEGA®
Fig. 7 Comparison of travel speed with single- and two-electrode VEGA®
Fig. 8 Comparison of amount of shielding gas consumption with singleand two-electrode VEGA®
reel, 20 kg spools, so it is necessary to replace the welding wire and
20 kg units. Therefore, the possible welding length without interruption on the 2-electrode VEGA® welding process is approximately
two times that of the single-electrode VEGA® welding process. When
welding is terminated, gouging occurs in that position. It is necessary to repair that using a shielded metal arc welding or a gas-shielded
metal arc welding. This translates into a greatly shortened amount
of processing time through the application of the 2-electrode VEGA®
welding process.
4.3 Reduction in the amount of shielding gas
Because the welding speed with this newly developed method is
substantially doubled, the amount of shielding gas consumed per
single welding length is approximately halved that for a single electrode. This again allows for a highly economical method of welding
(see Fig. 8).
4.4 Welding performance and application to ship building
The Mo-Ti-B type weld metal3-7), which is an alloy designed to
attain low temperature toughness that is essential in high heat input
welding, was also applied to the 2-electrode VEGA® welding process. Table 4 shows the materials and welding conditions used in the
test; Fig. 9 shows the groove face configuration; Fig. 10 shows the
locations of the test specimen. As can be seen in Table 5, performance was attained that met the standards of KEW53 and
KEW53Y40 as prescribed by the Nippon Kaiji Kyokai (Class NK).
See Photo 5 for a view of the microstructure of the weld metal (central location) on the steel plates. An extremely fine microstructure
was attained under high heat input welding. Also, mechanical performance was attained that had thorough mechanical property of HAZ
Table 4 Welding conditions
Plate
thickness
(mm)
Electrodes
Welding wire
1st
Flux-cored wire*1
2nd
1st
Solid wire*2
Flux-cored wire*1
Current
(A)
Voltage
(V)
Travel
speed
(cm/min)
Heat
input
(kJ/cm)
Electrode
spacing
(mm)
410
41
7.1
281
15
400
40
410
41
60
6.2
321
15
400
40
2nd
Solid wire*2
1st
Flux-cored wire*1
420
42
70
5.8
382
15
460
42
2nd
Solid wire*2
*1
: For two-electrode VEGA® (contains one-half the flux compared with the wire for single-electrode VEGA®)
*2
: JIS Z 3325 TGL1-4G (AP)
Base plate: EH40 (plate thickness: 70mm)
50
- 72 -
Shielding gas
Oscillation
width
(mm)
Composition
Flow rate
( /min)
5
100% CO2
30
17
100% CO2
30
35
100% CO2
30
NIPPON STEEL TECHNICAL REPORT No. 90 JULY 2004
of the applied EH40 steel.
In 2001, the 2-electrode VEGA® welding process was applied to
the welding of sheer strakes and hatch side coamings (plate thickness: 58 mm) of large sized container ships at Mitsubishi Heavy Industries Limited Kobe shipyard & Machinery Works (see Photo 6).
Also, in 2002, this method of welding was applied to hatch side
coamings of steel plates having thicknesses of 65 mm.
As described above, success was achieved in the development of
the 2-electrode VEGA® welding process which is a single-pass, and
highly efficient automatic welding method that attains stable penetration of weld metal even when vertically welding steel plates having thicknesses between 50 mm and 70 mm. This method was successfully applied the building of ultra large-sized container ships.
100µm
Photo 5 Microstructure of weld metal welded by two-electrode VEGA®
(center)
Fig. 9 Groove configuration
Photo 6 An example of construction site where two-electrode VEGA®
operated
5. Conclusions
According to the processes described above, success was attained
in the development of the 2-electrode VEGA® welding process as a
highly efficient automatic welding method for vertical position when
welding steel plates having thicknesses of 50 mm to 70 mm. The 2electrode VEGA® welding process realizes stable weld metal penetration and high efficiency in single-pass, vertical position welding
of steel plates having thicknesses between 50 mm and 70 mm, and
has obtained valuable tried and tested data through its applications
in actual container ship building. The scope of application to steel
plate thicknesses will expand in the future and can be expected to
become more widely applied because of its superior applicability as
a welding method.
Fig. 10 Sampling locations of test specimens
Table 5 Mechanical test results of weld metal with two-electrode VEGA®
Plate
thickness
Classification
(mm)
50
EH36
60
EH40
70
EH40
ClassNK KEW53
ClassNK KEW53Y40
0.2% proof stress
(MPa)
473
471
506
≥375
≥400
Tensile test
Tensile strength
(MPa)
610
611
656
490-660
510-690
- 73 -
Elongation
(%)
25
23
24
≥22
≥22
Charpy impact test
Absorbed energy at –20°C
(J)
89
86
79
≥34
≥41
NIPPON STEEL TECHNICAL REPORT No. 90 JULY 2004
References
Acknowledgements
1) Inagaki, M. et al.: Automatic Vertical Position Welding – Electroslag
Welding/Electrogas Welding. First edition. Tokyo, THE NIKKAN
KOGYO SHIMBUN Limited,1966, p.151
2) Japan Welding Sosiety: Welding and Joining Technology. First edition.
Tokyo, SANPO PUBLICATIONS Incorporated, 1993, p.388
3) Mori, N. et al.: Seitetsu Kenkyu. (307), 104(1982)
4) Mori, N. et al.: J. Jpn. Weld. Soc. 50(2), 174(1981)
5) Mori, N. et al.: J. Jpn. Weld. Soc. 50(8), 786(1981)
6) Ohkita, S. et al.: Australian Weld. J. 29(3), 29(1984)
7) Ohkita, S. et al.: ISIJ Int. 35(10), 1170(1995)
The authors wish to express their deep gratitude to the Mitsubishi
Heavy Industries Limited Kobe Shipyard & Machinery Works for
their cooperation in the development of the 2-electrode VEGA® welding process.
Note that the VEGA® welding process is a registered trademark
of Nippon Steel Welding Products & Engineering Company Limited.
- 74 -
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